How to use my logging class like a std C++ stream?

问题:

I've a working logger class, which outputs some text into a richtextbox (Win32, C++). Problem is, i always end up using it like this:

stringstream ss;  
ss << someInt << someString;  
debugLogger.log(ss.str());

instead, it would be much more convenient to use it like a stream as in:

debugLogger << someInt << someString;

Is there a better way than forwarding everything to an internal stringstream instance? If'd do this, when would i need to flush?

回答1:

You need to implement operator << appropriately for your class. The general pattern looks like this:

template <typename T>
logger& operator <<(logger& log, T const& value) {
    log.your_stringstream << value;
    return log;
}

Notice that this deals with (non-const) references since the operation modifies your logger. Also notice that you need to return the log parameter in order for chaining to work:

log << 1 << 2 << endl;
// is the same as:
((log << 1) << 2) << endl;

If the innermost operation didn't return the current log instance, all other operations would either fail at compile-time (wrong method signature) or would be swallowed at run-time.

回答2:

Overloading the insertion operator<< is not the way to go. You will have to add overloads for all the endl or any other user defined functions.

The way to go is to define your own streambuf, and to bind it into a stream. Then, you just have to use the stream.

Here are a few simple examples:

• Logging In C++ by Petru Marginean, DDJ Sept 05th 2007
• Rutger E.W. van Beusekom's logstream class, check also the .hpp alongside with this file

回答3:

As Luc Hermitte noted, there is "Logging In C++" article which describes very neat approach to solve this problem. In a nutshell, given you have a function like the following:

void LogFunction(const std::string& str) {
    // write to socket, file, console, e.t.c
    std::cout << str << std::endl;
}

it is possible to write a wrapper to use it in std::cout like way:

#include <sstream>
#include <functional>

#define LOG(loggingFuntion) \
    Log(loggingFuntion).GetStream()

class Log {
    using LogFunctionType = std::function<void(const std::string&)>;

public:
    explicit Log(LogFunctionType logFunction) : m_logFunction(std::move(logFunction)) { }
    std::ostringstream& GetStream() { return m_stringStream; }
    ~Log() { m_logFunction(m_stringStream.str()); }

private:
    std::ostringstream m_stringStream;
    LogFunctionType m_logFunction;
};

int main() {
    LOG(LogFunction) << "some string " << 5 << " smth";
}

(online demo)

Also, there is very nice solution provided by Stewart.

回答4:

An elegant solution that also solves the flushing issues is the following:

#include <string>
#include <memory>
#include <sstream>
#include <iostream>

class Logger
{
    using Stream = std::ostringstream;
    using Buffer_p = std::unique_ptr<Stream, std::function<void(Stream*)>>;

public:
    void log(const std::string& cmd) {
        std::cout << "INFO: " << cmd << std::endl;
    }

    Buffer_p log() {
        return Buffer_p(new Stream, [&](Stream* st) {
            log(st->str());
        });
    }
};

#define LOG(instance) *(instance.log())

int main()
{
    Logger logger;
    LOG(logger) << "e.g. Log a number: " << 3;
    return 0;
}

回答5:

In the Logger class, override the << operator.

Click Here to know how to implement the << operator.

You can also avoid the logging statements inside the code using Aspect Oriented programming.